Condensation nucleus counters (CNCs) provide the optical resolution necessary to detect and count sub-micron size particles by magnifying the size of the particles using condensate. CNCs operate on the principle of a cloud chamber. Vapor obtained by super-saturation of a gas derived from a working fluid, such as glycol, condenses upon sub-micron particles entrained in a sample gas, producing droplets on the order of a micron. In this fashion, the particles to be counted function as condensation nuclei for the vapor. The droplets, still entrained as sample gas, pass into a view volume of a particle counter operating according to light extinction or scattering principles. The droplets attenuate or scatter light in a beam of the particle counter, resulting in an appropriate output signal relative to particle condensation. The accuracy is dependent on the thermal and signal processing thresholds. The stream of fluent material, which includes droplets, passes through the optical beam, initiating the signal pulse. This flow region or view volume is defined by the optical and flow parameters of the detection system. Thus, some view volume characteristics can be controlled through nozzle and port design.
U.S. Pat. No. 5,118,959 to Caldow et al. discloses a CNC with a porous saturation below a condenser. An optical block containing a viewing volume is located above the condenser.
U.S. Pat. No. 5,239,356 to Hollander et al. discloses a condensation nucleus counter including a humidifier zone, a condensation zone extending orthogonally thereto and an optical detection system. The humidifying zone includes a duct and a hollow space extending parallel to the duct, but separated therefrom by a permeable material. A vaporizable liquid is received within the hollow space. The condensation zone includes a duct and a hollow space extending parallel thereto. Separating the duct from the hollow space is a permeable material. Particulate matter, entrained in sample air, is guided into the duct of the humidifying zone, where it is saturated. Upon passing through the condensation zone, the saturated air is supersaturated and condenses around the particulates.
U.S. Pat. No. 5,026,155 to Ockovic et al. discloses a process for sizing particles using condensation nucleus counting. The process employs a standard condensation nucleus counter in which the condensing temperature of a saturated working fluid is incrementally adjusted. In this manner, the sensitivity of the counter may be adjusted to afford discrimination of particles entrained in a gas, according to size.
U.S. Pat. No. 4,950,073 to Sommer discloses a sub-micron particle counter for counting particles entrained in air which includes a saturator, a condenser and an optical detector. Conduits extend through a bath of liquid contained in the saturator so that sample streams passing therethrough are heated by the bath. A flow divider separates the sample stream into a plurality of laminar flow streams, corresponding to the number of conduits in the bath. The condenser is a heat exchanger having a plurality of tubes with funnel-shaped inlet ends. The inlet ends are positioned over the bath of the saturator, with the tubes being inclined toward their inlet ends so that condensed vapor flows back into the bath.
U.S. Pat. No. 4,790,650 to Keady discloses a condensation nucleus counter including an inlet orifice leading to a flow path within a saturator. A condenser is in fluid communication with the saturator. Particulates entrained in a gas flow passing through the saturator have condensation accumulated thereon, forming a plurality of droplets. The plurality of droplets pass through an optical counter.
U.S. Pat. No. 4,449,816 to Kohsaka et al. discloses a system and a method of measuring hyperfine particles including a mixing chamber in fluid communication with both a saturated vapor chamber and a high temperature vapor chamber. An aerosol inlet is in constant fluid communication with the high temperature vapor chamber. The inlet is also in selective fluid communication with the saturated vapor chamber via a valve. An air aerosol containing fine particles is led into both the saturated vapor chamber and the high temperature vapor chamber, respectively, to produce saturated vapor aerosols. The two saturated vapor aerosols are led into the mixing chamber so that the aforementioned vapor is condensed on the aerosol particles. A particle detection station is included to measure the aerosol particles.
A problem encountered in prior art CNC devices is that vapor flow is carefully regulated until the flow enters the viewing volume. At this point, flow characteristics are largely neglected. However, view volume dynamics can play a key role in velocity profiling, false count elimination and contamination isolation. Yet turbulence expands the flow stream, removing some particles from view of the optics, and causing recirculation of other particles so that they are counted two or more times.
Another problem encountered in the prior art is that although CNC devices attempt to count small particles with great precision, they introduce vapor and particulate matter in the production environment where they are used. This occurs when vapor from the flow stream is exhausted and now, not only the particles entrapped in the vapor return to the ambient environment, but also the vapor. Where the working fluid in a vapor phase is not water, a new foreign substance is introduced into the ambient environment in small but measurable amounts.
An object of the invention was to devise a condensation nucleus counter having improved vapor flow, especially in the viewing volume, exhaust filtration and working fluid recovery.